<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JS的编写位置</title>
</head>
<body> 
    <script>
        const PI = 3.14
        let r = prompt('输入圆的半径')
        let s = PI*r*r
        document.write('圆的面积：'+s)
        // document.write(s)
        </script>
</body>
</html>